The four turning points of the year, two solstices and two equinoxes, are the skeleton of our seasons. They explain why June days in London are long and December days are short, why Australians celebrate Christmas in summer, and why the Sun rises in a different place on the horizon as the months pass. This guide to solstices and equinoxes explains what they are, how Earth’s tilt produces them and what each one means in practice.
The one idea behind it all: Earth’s tilt
Earth spins on an axis that is tilted about 23.4 degrees from the perpendicular to its orbital plane. Throughout its year-long orbit, the axis keeps pointing in nearly the same direction in space, toward a point close to the star Polaris in the north.
That steady tilt is the whole story. As Earth travels around the Sun:
- For part of the year the Northern Hemisphere leans toward the Sun, getting more direct light and longer days.
- Six months later the Southern Hemisphere leans toward the Sun instead.
- In between, neither hemisphere leans toward or away.
Seasons do not come from changes in Earth’s distance from the Sun. Earth is actually nearest the Sun in early January, when the Northern Hemisphere is in winter. The tilt matters because it changes the angle at which sunlight hits the ground and how long the Sun stays above the horizon. Higher Sun angles concentrate energy on a smaller area, and longer days give more hours of heating.
The four turning points
| Event | Approximate date | What happens | Northern Hemisphere | Southern Hemisphere |
|---|---|---|---|---|
| March equinox | 19 to 21 March | Sun crosses the equator heading north | Spring begins | Autumn begins |
| June solstice | 20 to 22 June | Sun at its northernmost point | Summer begins, longest day | Winter begins, shortest day |
| September equinox | 22 to 24 September | Sun crosses the equator heading south | Autumn begins | Spring begins |
| December solstice | 20 to 23 December | Sun at its southernmost point | Winter begins, shortest day | Summer begins, longest day |
The exact moment varies from year to year. For this year’s dates, see key equinox and solstice dates for 2026.
What is a solstice?
The word solstice comes from Latin sol (Sun) and sistere (to stand still). Around the solstice, the Sun’s noon height in the sky stops climbing or falling for a few days, then reverses. It is a turning point.
At the June solstice, the Sun is directly overhead at noon at the Tropic of Cancer, at about 23.4 degrees north latitude. At the December solstice, it is directly overhead at the Tropic of Capricorn, at about 23.4 degrees south.
The summer solstice
The summer solstice is the day with the most daylight in a hemisphere. The Sun reaches its highest point in the sky at noon, rises far to the north-east (in the Northern Hemisphere) and sets far to the north-west. The farther you are from the equator, the larger the effect:
- At the equator, day length hardly changes during the year.
- At mid-latitudes, the longest day may be many hours longer than the shortest.
- North of the Arctic Circle, around 66.6 degrees north, the Sun does not set for at least one day around the June solstice, the phenomenon often called the midnight Sun.
The winter solstice
The winter solstice is the day with the least daylight. The Sun is lowest at noon, and rises south-east and sets south-west in the Northern Hemisphere. Inside the Arctic Circle, the Sun does not rise for at least one day, called polar night. The Southern Hemisphere experiences these in reverse around the December solstice.
Counterintuitively, the coldest weather does not arrive on the winter solstice but weeks later, because land and ocean continue to lose heat for a while after days start lengthening. This delay is known as seasonal lag.
What is an equinox?
At an equinox, the Sun is directly above the equator, and day and night are close to equal in length. The Sun rises almost due east and sets almost due west. The word comes from Latin for “equal night.” For more detail on this event, see Equinox Explained.
How day length changes through the year
Daylight does not change at a steady pace. It changes fastest around the equinoxes and slowest around the solstices, a pattern that follows the shape of a sine curve.
At mid-latitudes, a few weeks before and after a solstice, the days seem to hardly change, while around the equinoxes you may gain or lose several minutes of daylight every day. This is why people often notice that spring evenings brighten quickly in March but December feels stuck in the dark for weeks.
Latitude changes the size of the swing. If you want to calculate it for your location, our guide to day length around the solstice shows the method.
Why the seasons are opposite north and south
Because the hemispheres lean in opposite directions, when it is summer in one it is winter in the other. This is why the June solstice is the summer solstice in the north and the winter solstice in the south. Some organizations use “June solstice” and “December solstice” in international contexts to avoid confusion.
Near the equator, the seasons are less defined by temperature and more by rainfall patterns, such as wet and dry seasons. The tilt still matters, but the Sun is high all year.
Two ways to define seasons
- Astronomical seasons start at the equinoxes and solstices. They are based on Earth’s position in orbit.
- Meteorological seasons group the year into three-month periods based on temperature records: in the Northern Hemisphere, winter is December to February, spring March to May, summer June to August and autumn September to November.
The two systems differ by about three weeks at the start of each season. Both are legitimate.
Are the seasons the same length?
Not quite. Earth’s orbit is slightly elliptical, and Earth moves faster when closer to the Sun. As a result, the seasons differ by a few days in length. In the Northern Hemisphere, spring and summer are slightly longer than autumn and winter.
How people have marked these days
Observing the Sun’s turning points has been central to human culture.
- Monuments and alignments. Some ancient sites around the world appear aligned to solstice or equinox sunrises or sunsets. Stonehenge in England is a famous example for the solstices, and many visitors gather there around these dates.
- Festivals. Midsummer celebrations in Northern Europe, winter festivals of light, and New Year festivals such as Nowruz are connected in different ways to the solar year.
- Agriculture. Farmers used the solar calendar to decide when to sow and harvest.
- Religion and calendars. The timing of some religious observances is connected to the solar year.
- Modern practice. Many people mark them with walks, bonfires, gatherings, meditation, or simply by noticing the light.
What you can observe yourself
You do not need a telescope to track the seasons.
- Watch the sunrise point. From a fixed spot, note where the Sun rises on the horizon throughout the year. It moves north from December to June and south from June to December (in the Northern Hemisphere).
- Record noon shadows. Place a vertical stick in level ground and mark the end of its shadow at local solar noon. The shadow is shortest around the summer solstice and longest around the winter solstice.
- Log sunrise and sunset times from an almanac and plot them on a graph.
- Take a photo from the same place each month to see the Sun’s height and the foliage change.
Safety note: never look directly at the Sun. Do not use binoculars or telescopes on it without certified solar filters.
If you want to try the equinox version of this, read how to observe sunrise and sunset on the equinox.
Teaching the seasons
The tilt idea is easy to demonstrate with a lamp and a ball tilted on a stick, showing how the same lamp lights the surface at different angles. For a classroom or family activity, see how to explain Earth’s axial tilt to kids.
Common misconceptions
“Summer is when Earth is closest to the Sun.” In the Northern Hemisphere it is the opposite: Earth is nearest in early January.
“The solstice is the hottest or coldest day.” The temperature peaks and lows lag behind by weeks.
“The solstice lasts a day.” It is a moment in time, though we mark the day.
“The seasons start on the same date every year.” The dates vary by a day or so because the calendar and the solar year do not align perfectly.
“All places get 12 hours of daylight at the equinox.” It is close to 12, but not exact, as the Sun’s size and atmospheric refraction add several minutes.
Frequently asked questions
What is the difference between an equinox and a solstice?
At an equinox, day and night are close to equal. At a solstice, one is at its extreme: the longest day or the shortest day of the year.
Why is the summer solstice not the hottest day?
The land and oceans take time to heat up. Peak temperatures typically arrive weeks after the solstice.
Do solstices and equinoxes happen at the same time everywhere?
The instant is the same worldwide, but the local clock time and date depend on your time zone.
What happens at the poles?
Each pole has about six months of continuous daylight and six months of continuous darkness, roughly, with twilight on either side. The transitions happen around the equinoxes.
Equinox Explained: What Happens and Why It Matters
Key Equinox and Solstice Dates for 2026
Conclusion
Solstices and equinoxes are the four markers of Earth’s journey around the Sun, all caused by a constant axial tilt of about 23.4 degrees. Equinoxes bring nearly equal day and night, while solstices bring the longest and shortest days. The next time the seasons turn, step outside at sunrise or sunset and see for yourself how the Sun’s path has shifted.
